Re: Measuring the varying Resistance of a DC motor


Rich256 wrote:
bill.sloman@xxxxxxxx wrote:
Rich256 wrote:
troyrivera@xxxxxxxxx wrote:
I have a DAQ that uses LABVIEW. I need to calculate the changing
current going to a DC motor. I cannot measure the current directly
because it is too high for my DAQ to handle. Basically I need to
continuously monitor the varying resistance of the motor, so that I can
calculate the current the motor is pulling. Does anybody have any idea
how I could do this? I was considering breaking open an ohmmeter, but
I'm not sure how an ohmmeter works. Does the ohmmeter send a certain
voltage with certain resistances?

Thanks in advance

It isn't motor resistance that changes. It's back emf. As a motor is
loaded the back emf decreases, drawing more current.

Motor resistance does change, as the coils heat up.

Back-emf doesn't depend on load, but on motor speed and - to a lessor
extent - motor temperature. The magnetic field generated by a permanent
magnet decreases as the magnet gets hotter, and with it the back-emf
per unti motor speed.



Which comes first, load or speed?

Back emf is created by the 'moving motor winding cutting the magnetic
field generted by the fixed magnet/excitation coils. Speed thus coomes
first.

If you've got a servo-controlled motor that maintains the same speed,
you see the same back emf independent of the torque being generated.

Put on a load and the motor slows
etc, etc. I would think the motor resistance change is negligible.

Small - +0.3% per degree Celecius.

Stall a motor and it probably burns due to lack of back emf.

No, it burns out due to escessive curren through the coil, which
intelligent driver design can prevent.

--
Bill Sloman, Nijmegen

.

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